Protein kinases (“PKs”) are enzymes that catalyze the phosphorylation of hydroxy groups on tyrosine, serine and threonine residues of proteins. The consequences of this seemingly simple activity are staggering; cell growth, differentiation and proliferation, i.e., virtually all aspects of cell life in one way or another depend on PK activity. Furthermore, abnormal PK activity has been related to a host of disorders, ranging from relatively non-life threatening diseases such as psoriasis to extremely virulent diseases such as glioblastoma (brain cancer).
Generally, the PKs can be categorized into two classes, the protein tyrosine kinases (PTKs) and the serine-threonine kinases (STKs). However, other kinases are reported that phosphorylate other amino acids, such as histidine. Kinases with dual (serine/threonine and tyrosine) specificity are also reported (e.g., MEK or MAPKK).
Many PTKs are involved with growth factor receptors. When bound by a growth factor ligand, growth factor receptors are converted to an active form which interacts with proteins on the inner surface of the cell membrane. This leads to phosphorylation on tyrosine residues of the receptor and other proteins and to the formation inside the cell of complexes with a variety of cytoplasmic signaling molecules that, in turn, affect numerous cellular responses such as cell division (proliferation), cell differentiation, and cell growth.
Growth factor receptors with PTK activity are known as receptor tyrosine kinases (“RTKs”). They comprise a family of transmembrane receptors with diverse biological activity. The HER subfamily of RTKs includes EGFR (epithelial growth factor receptor), HER2, HER3 and HER4. These RTKs consist of an extracellular glycosylated ligand binding domain, a transmembrane domain and an intracellular cytoplasmic catalytic domain that can phosphorylate tyrosine residues on proteins.
Another RTK subfamily consists of insulin receptor (IR), insulin-like growth factor I receptor (IGF-1R) and insulin receptor related receptor (IRR). IR and IGF-1R interact with insulin, IGF-I and IGF-II to form a heterotetramer of two entirely extracellular glycosylated alpha subunits and two beta subunits which cross the cell membrane and which contain the tyrosine kinase domain.
A third RTK subfamily is referred to as the “platelet derived growth factor receptor” (“PDGFR”) group, which includes PDGFR-α, PDGFR-β, CSFIR, c-kit and c-fms. These receptors consist of glycosylated extracellular domains composed of variable numbers of immunoglobin-like loops and an intracellular domain wherein the tyrosine kinase domain is interrupted by unrelated amino acid sequences.
Another group, which, because of its similarity to the PDGFR subfamily (and is sometimes subsumed into the PDGFR subfamily) is the fetus liver kinase (“flk”) receptor subfamily. This group is believed to be made up of kinase insert domain-receptor fetal liver kinase-1 (KDR/FLK-1, VEGF-R2), flk-1R, flk-4 and fins-like tyrosine kinase 1 (flt-1).
A further member of the tyrosine kinase growth factor receptor family is the fibroblast growth factor (“FGF”) receptor subgroup. This group consists of four receptors, FGFR1-4, and seven ligands, FGF1-7. While not yet well defined, it appears that the receptors consist of a glycosylated extracellular domain containing a variable number of immunoglobin-like loops and an intracellular domain in which the tyrosine kinase sequence is interrupted by regions of unrelated amino acid sequences.
Still another member of the tyrosine kinase growth factor receptor family is the vascular endothelial growth factor (“VEGF”) receptor subgroup. VEGF is a dimeric glycoprotein similar to PDGF but has different biological functions and target cell specificity in vivo. In particular, VEGF is presently thought to play an essential role is vasculogenesis and angiogenesis.
In addition to the RTKs, there also exists a family of entirely intracellular PTKs called “non-receptor tyrosine kinases” or “cellular tyrosine kinases” (CTKs). CTKs do not contain extracellular and transmembrane domains. More than 24 CTKs in 11 subfamilies (Src, Frk, Btk, Csk, Abl, Zap70, Fes, Fps, Fak, Jak and Ack) have been identified. The Src subfamily appears so far to be the largest group of CTKs and includes Src, Yes, Fyn, Lyn, Lek, Bik, Hck, Fgr and Yrk.
The serine/threonine kinases, STKs, like the CTKs, are predominantly intracellular although there are a few receptor kinases of the STK type. STKs are the most common of the cytosolic kinases.
RTKs, CTKs and STKs have all been implicated in a host of pathogenic conditions including, significantly, various cancers. Other pathogenic conditions which have been associated with PTKs include, without limitation, psoriasis, hepatic cirrhosis, diabetes, angiogenesis, restenosis, ocular diseases, rheumatoid arthritis and other inflammatory disorders, immunological disorders such as autoimmune disease, cardiovascular disease such as atherosclerosis and a variety of renal disorders.
With regard to cancer, two of the major hypotheses advanced to explain the excessive cellular proliferation that drives tumor development relate to functions known to be PK regulated. It has been suggested that malignant cell growth results from a breakdown in the mechanisms that control cell division and/or differentiation. It has been shown that the protein products of a number of proto-oncogenes are involved in the signal transduction pathways that regulate cell growth and differentiation. These protein products of proto-oncogenes include the extracellular growth factors, transmembrane growth factor PTK receptors (RTKs), cytoplasmic PTKs (CTKs) and cytosolic STKs, discussed above.
Many small molecule PTK inhibitors have been developed over the years, including, but not limited to, imatinib, dasatinib, canertinib, erlotinib, gefitinib, lapatinib, sorafenib, sunitinib, and vatalinib. These molecules have been prescribed for many diseases, including, chronic myelogenous leukemia (CML), gastrointestinal stromal tumors (GISTs), renal cell carcinoma, and solid tumors, including breast, lung, and colorectal cancers; and are used as anti-neoplastic agents and as radio-sensitizing agents. However, treatment with these agents suffer from many side effects, including, hypertension, fatigue, asthenia, diarrhea, hand-foot syndrome, neutropenia and myelosuppression, peripheral edema, headache, and hypocalcemia.
Therefore, pharmacotherapy with such therapeutic PTK inhibitors would be improved if these and/or other adverse or side effects associated with their use could be decreased or if their pharmacology may be improved. Thus, there is a large unmet need for developing novel PTK inhibitor compounds.
The present invention seeks to address these and other needs in the art.